"One of my colleagues was yelling out, 'Stop, stop, stop' pretty loud, and we all sort of looked to see what was going on, and we looked over the ledge on to the road, and the concrete truck was dragging the lady along the floor," he said.

Police investigators are piecing together what happened in the fatal accident on Sydney's North Shore. (Nine)

A large part of Willoughby Road was shut down for most of the day as crash investigators worked out exactly what happened.

Investigators are focusing on who had the right of way at those traffic lights and whether the truck driver had seen the elderly couple before he started turning.